Formula Writing Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Formula Writing.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The systematic process of combining element symbols and numerical subscripts to represent the exact composition of a chemical compound, ensuring that the total positive charge.

Chemical formulas are the 'spelling' of chemistry — they tell you exactly which atoms and how many of each are in a compound.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: In ionic compounds, the total positive charge must equal the total negative charge. The subscripts are the smallest whole numbers that achieve this.

Common stuck point: The subscripts tell you the ratio of atoms, not the charge. CaCl₂ means 1 calcium and 2 chlorines, not calcium with a charge of 2.

Sense of Study hint: When writing ionic compound formulas, use the criss-cross method. First write the symbols of the cation and anion with their charges. Then criss-cross the charge numbers to become subscripts on the opposite ion (drop the signs). Finally, reduce the subscripts to the smallest whole-number ratio and verify that the total positive charge equals the total negative charge.

Worked Examples

Example 1

easy
Write the chemical formula for aluminum chloride using the charges of the ions: \text{Al}^{3+} and \text{Cl}^-.

Solution

  1. 1
    Aluminum has a charge of +3 and chlorine has a charge of -1.
  2. 2
    To balance the charges, we need 3 chloride ions for every 1 aluminum ion: 3 \times (-1) = -3 balances +3.
  3. 3
    The formula is \text{AlCl}_3. The subscript 3 on Cl indicates three chloride ions per formula unit.

Answer

\text{AlCl}_3
When writing ionic compound formulas, the total positive charge must equal the total negative charge so the compound is electrically neutral. The crisscross method (using charge magnitudes as subscripts) is a quick shortcut.

Example 2

medium
Write the chemical formula for calcium phosphate using the crisscross method. Calcium forms \text{Ca}^{2+} and phosphate is \text{PO}_4^{3-}.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
Write the correct chemical formulas for: (a) iron(III) sulfate, (b) ammonium carbonate.

Example 2

hard
A student writes the formula for magnesium nitride as \text{MgN}. Explain why this is incorrect and determine the correct formula. (\text{Mg}^{2+}, \text{N}^{3-})
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Background Knowledge

These ideas may be useful before you work through the harder examples.

ionchemical bond